Modern integrated circuits are formed on semiconductor chips. To increase manufacturing throughput and lower manufacturing cost, the integrated circuits are manufactured in semiconductor wafers, each containing many identical semiconductor chips. After the integrated circuits are made, semiconductor chips are sawed from the wafers and packaged before they can be used.
In typical packaging processes, semiconductor chips (also referred to as dies in the art) are first attached to package substrates. This includes physically securing the semiconductor chips on the package substrates, and connecting bonding pads on the semiconductor chips to bonding pads on the package substrates. Underfill, which typically comprises epoxy, is used to secure the packages. The semiconductor chips may be bonded using either flip-chip bonding or wire bonding. The packages are then attached to printed circuit boards (PCBs) through ball grid array (BGA) balls. Typically, BGA balls are mounted on the pads of the package substrates, and then the packages are mounted on PCBs through the BGA balls.
To make reliable electrical contacts between the package substrates and the PCBs, the BGA balls preferably have a good coplanarity, so that all BGA balls may have good contacts with the PCBs. However, the package substrates typically have warpage. In addition, during the packaging processes, such as flip-chip bonding, underfill dispensing and ball mounting are performed at elevated temperatures, and thus further worsens the warpage of the package substrates. Furthermore, BGA balls may vary in size. Therefore, the BGA balls are not naturally coplanar. This results in assembly yield loss due to the non-contact between BGA balls and PCBs.
FIG. 1 schematically illustrates a package having non-coplanar BGA balls. The package includes chip 2 bonded to package substrate 4. BGA balls 6 are mounted on package substrate 4. Assuming package substrate 4 has warpage, and the center portion is lower than edge portions, resulting in center BGA ball 6-1 being lower than edge ball 6-2.
FIG. 2 illustrates center ball 6-1 and edge ball 6-2. It reveals that due to the warpage of package substrate 4, there is a height difference D between center ball 6-1 and edge ball 6-2. When the package is mounted on a PCB board, edge ball 6-2 will have good contact with the PCB board, while center ball 6-1 is likely to be out of contact with the PCB board, and thus the resulting package fails.
Typically, it is preferred that for package substrates smaller than 37.5 mm×37.5 mm, the coplanarity, which measures a maximum distance at a point on the package substrate and allowed to deviate from a plane of other points, is less than about 6 mils. For package substrates larger than 37.5 mm×37.5 mm, the coplanarity is preferably less than about 8 mils. However, in conventional packaging practices, typically several percentages of the packages have warpage beyond these ranges, and hence resulting in yield loss. The problem worsens with the enlargement of the package substrates, which are increasingly used, for example, in system-in-package (SIP).
Accordingly, what is needed in the art is a method for solving the non-coplanarity problem of package substrates, and hence to increase production yield.